Market adoption of wireless LAN (WLAN) technology has exploded, as users from a wide range of backgrounds and vertical industries have brought this technology into their homes, offices, and increasingly into the public air space. This inflection point has highlighted not only the limitations of earlier-generation systems, but also the changing role that WLAN technology now plays in people's work and lifestyles across the globe. Indeed, WLANs are rapidly changing from convenience networks to business-critical networks. Increasingly users are depending on WLANs to improve the timeliness and productivity of their communications and applications, and in doing so, require greater visibility, security, management, and performance from their network.
Spectrum regulation has required that transmitters operating in the so-called “radar channels” (5.25 to 5.75 GHz) perform a “channel availability check” prior to commencing transmissions on that channel. A channel availability check typically involves monitoring a candidate channel for 60 seconds to determine if radar transmissions are present. Typically, wireless access points perform this function as part of their initialization process. Because radar transmissions may appear at any time, wireless access points are required to continually monitor the spectrum, in a process referred to as “in-service monitoring.” As spectrum regulation has given radar transmissions priority in the radar channel band, once radar transmissions are detected, wireless access points and their associated wireless clients must cease transmissions on that channel so as not to interference with radar operations.
Wireless clients are also required to perform a channel availability check prior to transmitting. Because wireless clients are mobile, they routinely roam between wireless access points covering adjacent areas in a wireless network. In 802.11 networks, wireless clients may either perform passive scanning or active scanning to determine the next wireless access point to which to associate. Wireless clients performing passive scanning scan just-in-time to receive a beacon transmission from another wireless access point if they know when the transmission will occur. Otherwise, they may have to wait for up to 100 ms, the typical beacon period, for the beacon to be transmitted assuming an access point is transmitting on the channel; if no AP is transmitting on this channel, wireless clients could potentially wait even longer. For this reason, active scanning, where a client transmits probes one or more channels, is generally preferred. However, wireless clients cannot perform an active scan on a channel without either performing a channel availability check or receiving an enabling signal from a wireless access point. Delays incurred by wireless clients while scanning can negatively impact on-going real-time traffic such as a voice or video call if they lead to an interruption in connectivity to an access point.
The channel availability check is performed for a continuous period of time using passive scanning procedures. If, while performing the channel availability check, the wireless client receives a beacon frame transmitted by a wireless access point on that channel, the client is deemed to have received an acceptable “enabling signal.” Upon receipt of the beacon, the client may commence actively scanning the candidate channel.